1. Field of the Invention
The present invention relates to a method for manufacturing a boundary acoustic wave device utilizing a boundary acoustic wave propagating through an interface between first and second solid media and relates to a boundary acoustic wave device. More specifically, the present invention relates to a boundary acoustic wave device manufacturing method including a step of adjusting frequency characteristics and relates to a boundary acoustic wave device.
2. Description of the Related Art
A surface acoustic wave device is widely used as a band-pass filter or a resonator, for example, in a cellular phone. In a surface acoustic wave device, an interdigital transducer (IDT) electrode is disposed on a piezoelectric substrate, and the IDT electrode receives and excites a surface acoustic wave. Accordingly, in order to avoid interference with reception and excitation of a surface acoustic wave, it is necessary to have a space above a portion where the IDT electrode is disposed.
When a surface acoustic wave device is used as a band-pass filter or a resonator, the frequency of the surface acoustic wave device must be controlled with high accuracy. Accordingly, after the surface acoustic wave device is produced, a mass load member may be provided to the piezoelectric substrate or the electrode may be processed for the purpose of adjusting the frequency.
Japanese Unexamined Patent Application Publication No. 2001-77661 discloses a method by which a frequency can be adjusted after a surface where an IDT electrode of a surface acoustic wave device is disposed is sealed with a cap. FIG. 12 is a schematic front cross-sectional view for describing the frequency adjusting method described in Japanese Unexamined Patent Application Publication No. 2001-77661. It is noted that hatching indicating a cross section is omitted in this drawing, which schematically illustrates the cross section.
According to the frequency adjusting method described in Japanese Unexamined Patent Application Publication No. 2001-77661, a surface acoustic wave device 501 includes a piezoelectric substrate 502, an IDT electrode 503 disposed on the piezoelectric substrate 502, and a cap member 504 attached above the piezoelectric substrate 502 such that a space S is present above the IDT electrode 503. That is, an adhesive 505 is applied to seal the space S and surround a portion where the IDT electrode 503 is disposed, and the cap member 504 is bonded to the piezoelectric substrate 501 by the adhesive 505.
Accordingly, it is difficult to adjust the frequency by processing the portion where the IDT electrode 503 is disposed after the surface acoustic wave device 501 is produced. However, in Japanese Unexamined Patent Application Publication No. 2001-77661, the frequency is adjusted after the cap member 504 is bonded to the piezoelectric substrate 501. Here, a substance 506 that is vaporized by being heated by a laser beam is applied on the inner surface of the cap member 504 in advance. The cap member 504 is made of a light-transmitting material that allows a laser beam to pass therethrough. When the surface acoustic wave device 501 is irradiated with a laser beam emitted from thereabove using a laser apparatus 507, the laser beam transmits through the cap member 504. Accordingly, the substance 506 is heated by the laser beam, vaporized, and deposited on the IDT electrode 503 positioned therebelow. By controlling the irradiation energy of this laser beam, it is possible to adjust the frequency, according to Japanese Unexamined Patent Application Publication No. 2001-77661.
Nowadays, attention is being given to a boundary acoustic wave device, instead of a surface acoustic wave device. For example, WO 2004/070946 discloses a boundary acoustic wave device that includes an IDT electrode arranged at an interface between first and second solid media and that uses a boundary acoustic wave propagating through the interface. In this boundary acoustic wave device, the IDT electrode is arranged at the interface between the first and second solid media, and there is no need to have a space to which the IDT electrode is exposed.
It is unnecessary for a boundary acoustic wave device to have a space to which an IDT electrode is exposed. Accordingly, a boundary acoustic wave device can be more compact when compared with a surface acoustic wave device and can have a simplified package structure.
When a boundary acoustic wave device is used as a filter or a resonator, it is also necessary to set its frequency with high accuracy. However, in the boundary acoustic wave device, an IDT electrode is embedded at an interface between first and second solid media and there is no space to which the IDT electrode is exposed. Accordingly, the frequency adjusting method for use in the surface acoustic wave filtering device described in Japanese Unexamined Patent Application Publication No. 2001-77661 is inapplicable to the boundary acoustic wave device.
That is, in the surface acoustic wave device, there is no second medium above the IDT, and the surface portion of the IDT and the piezoelectric substrate, which allow an elastic wave to pass therethrough, faces a space. Reforming to change the acoustic velocity of the surface portion of the IDT and the piezoelectric substrate can be performed from the space side. If such frequency adjustment is applied to a boundary acoustic wave device, a portion of the second medium that is opposite to the interface to the first medium would be reformed, and the second medium positioned at the interface through which a boundary acoustic wave propagates could not be reformed.
Accordingly, traditionally, it has been necessary to manufacture a boundary acoustic wave device that can exhibit desired characteristics with high accuracy in manufacturing thereof. There is no known effective method for adjusting frequency characteristics after manufacturing.